The traditional broadband light sources for optical spectroscopy in the near UV, visible, and/or near-infrared wavelengths are the fluorescent, incandescent, and arc-lamp bulbs. Typically, a spectroscopy bulb is optically coupled to the test sample via gratings, lenses, fibers, and/or free-space transfer. However, such traditional light sources have significant native disadvantages, including that: (a) they produce their light rather inefficiently, wasting a large proportion of the power supplied to them as heat and unusable wavelengths of light. This is a drawback in devices where significant local heating (such as medical devices in contact with tissue) or high power consumption (such as battery operated devices for field use) are undesirable or unacceptable; and (b) they emit light over a wide spherical angle from non-point sources, rendering inefficient any attempts to direct their light either onto spectroscopy samples (such as living tissue) or into optical delivery systems (such as fibers coupled to test samples), which in turn further raises heat production and power consumption for any desired level of sample illumination.
These limitations are best appreciated by example. First, with specific regard to the production of large amounts of heat, conventional bulbs are inefficient at best. The visible light output from a conventional incandescent light bulb represents only 4% of the total power consumed by the bulb. This conversion efficiency rises to only 14% for so-called high-output halogen lamps (though the improved efficiency results in accelerated drift and bulb aging). These efficiencies can easily drop farther, by a factor of 5 or more, if one considers only in-band light used in spectroscopic analysis (e.g., a 500–600 nm light band for hemoglobin analysis) in determination of the conversion efficiency.
The physical reason for this meager rate of energy conversion is that tungsten filaments, as well as heated arc lamp electrodes, operate as blackbody thermal radiators, and thus radiate mostly infrared radiation, plus a small component of UV radiation, at any temperature they can withstand. While in theory an ideal blackbody radiator produces visible light most efficiently at 6,600K (11,500° F.), nothing known in the art remains solid for use as a filament at this temperature, which exceeds the temperature at the sun's surface. Even so-called “high-efficiency” projection-type halogen lamps must therefore operate far below this ideal temperature, often operating instead from 2,700K to 3,500K (just below Tungsten's melting point of 3,683K). As a result, such bulbs typically require 6.9 W of power to produce 5.9 W of heat, in addition to 1.0 W of light.
Such poor conversion efficiencies result in a high degree of unwanted thermal output, making conventional bulbs run hot, and in turn preventing close illumination of the sample and often relegating bulb-based light sources into fiber-coupled hot, external, fan-cooled boxes.
Second, with regard to the broad spatial emission, conventional bulbs typically produce light in all directions in the absence of mirrors or lenses—that is, relatively uniformly over a full 4π spherical angle. Further, because a filament has a length and a width, the light can no longer be focused to a point. This broad spatial emission typically makes a direct coupling of light from a conventional bulb onto a sample, or into an optical guide, inefficient. For illustration, consider a 1 cm diameter spherical bulb in which light from the bulb's filament radiates evenly in all directions. The glass surface resides approximately 5 mm from the filament in all directions, for a surface area of the glass sphere of 4/3*π*r2, or 105 mm2. The portion of this uniform field of radiated light reaching a 1 mm diameter sample, placed up against the bulb glass, measures only 0.79 mm2. Thus, this tiny sample intercepts (and is thus illuminated by) only 0.2% of the total light output from the bulb, as given by the ratio (0.79 mm2/105 mm2), with 99.8% of the bulbs output wasted. The less compact a lamp's source, the more difficult it becomes to focus and guide its light. This is especially true for UV fluorescent lamps, where focusing losses are far higher than for a halogen bulb.
Further, the surface temperature of a halogen bulb often exceeds 120° C., making a close approximation of a hot bulb and sample not wise or practical in many cases, especially if the sample is living or fragile. Moving the sample away from the bulb, in order to spare the sample from heating, only worsens the inefficiencies described above. Nor is the situation improved by separating the bulb and sample using optical fiber. Directly attaching an optical fiber to the glass or quartz surface of 1 cm diameter bulb discussed above (such as by using optical glue) allows the fiber to intercept and capture only those photons striking the face of the fiber. A fiber measuring only 100 microns in diameter has a tiny face area measuring just 0.0079 mm2. Thus, a 100 micron fiber, glued to the bulb 5 mm from the filament, collects only 0.002% of the bulb's emitted light, as given by the ratio (0.0079 mm2/105 mm2). Even if the diameter of the fiber in this example were to be enlarged 10 fold, this transfer ratio would rise to only 0.2% of the bulb's visible light output that is intercepted and transmitted to the sample, again with 99.8% of the bulbs output lost and wasted.
All told, when taking into consideration both of the above limitations, the poor conversion efficiency of energy to light and the poor transfer efficiency of light to the sample, only 0.0003% of the energy flowing into the 1 cm bulb discussed above ends up converted to visible light, captured, and successfully transmitted by a fiber to a tissue sample, for more than 99.9997% of the total light wasted. Here, we term the 0.0003% figure of merit the delivered efficiency. Another way of expressing how poor this net delivered efficiency is, in fact, is that for the preceding bare-fiber-to-bare-bulb example, 369,524 watts of energy would have been required by the bulb for each watt of light delivered to sample or tissue, with the remainder released and lost as heat. These limitations of conventional sources are apparent in the art.
Broadband lamp sources or lamp designs are known, and are used for spectroscopy. Most art regarding illumination sources for spectroscopy suggest devices or methods that describe conventional light sources, although some describe more exotic lamp sources (e.g., U.S. RE29,304). White LEDs are known (e.g., U.S. Pat. No. 6,252,254, WO 01/01070), however none are suggested as spectroscopic light sources, and their high conversion efficiency, narrow angle of emission, and optical stability have not been cited nor exploited for spectroscopy purposes, especially in medicine for in vivo uses, save merely that they have been mentioned in passing for the purpose of general non-spectroscopic endoscopic illumination (U.S. Pat. No. 6,251,068). Several schemes for reducing heat production or for transmitting light to a sample are known (e.g., such as light conducting rods in U.S. Pat. No. 5,974,210), but none with the purpose of improving the efficiency of delivery, nor are these sources specifically designed to operate as cool spectroscopic illuminators with high delivery efficiency.
With specific regard to medical probes coupled to or embedded with light sources, a number of systems are known. Examples include invasive or tissue surface monitoring devices equipped with fiber optics, such as catheters, needles, and trocars (e.g., U.S. Pat. No. 5,280,788, U.S. Pat. No. 5,931,779), as well devices containing the light source itself (e.g., U.S. Pat. No. 5,645,059, U.S. Pat. No. 5,941,822, WO 00/01295). These systems typically completely ignore the complex issues of broadband illumination source design, suggesting only that known or existing light sources can be used rather than proposing improved illumination sources, and none of these systems consider specifically design issues regarding design and deployment of broadband optical light sources, especially with regard to conversion efficiency, source efficiency, and heat transfer to the sample, nor do they propose any specific or novel high delivery efficient optical sources.
Therefore, all of the above illumination systems and methods suffer from one or more limitations noted above, in that they are either narrow band emitters (such as lasers or filtered spectra), they are not configured to deliver light with a high efficiency, they have obligatory high local heating of the sample, they couple relatively poorly between the bulb and tissue or sample, they are not appropriate to be placed in close proximity to samples, and/or they ignore or omit design considerations regarding illumination efficiency and density, and thus fail to reliably provide an improved illumination source for the performance of spectroscopy in thermally sensitive samples, such as living tissues, or in spatially constrained geometries, such as through fibers and needles.
None of the above systems suggest or teach a method and system to more efficiently deliver light to tissue or spectroscopy samples without damaging delicate samples, in order to produce a more efficient and/or high density illumination for the performance of spectroscopy in thermally sensitive samples, such as living tissues, or in spatially constrained geometries, such as through fibers and needles, such as for identifying tissue by type or state or for monitoring the oxygenation of living tissues, in vivo and in real time. A delivery-optimized, reduced heat, high-density illuminator for real-time in vivo spectroscopic applications has not been taught, nor has such a tool been successfully commercialized.